JavaScript Basic

Question 1

What is JavaScript

Answer 1

a scripting or programming language that allows you to implement complex features on web page

Question 2

Java vs JavaScript

Answer 2

Java code must be compiled, and JavaScript code is interpreted directly by a borwser
JavaScript code is run on a browser only, while Java creates applications that run in a virtual machine or browser.
Java is an OOP (object-oriented programming) language, and JavaScript is specifically an OOP scripting language.
Java Multithreaded, JavaScript is single threaded
Java is static type and JavaScript is dynamic type

Question 3

Advantage of JavaScript

Answer 3

Client side no need for server

Dynamically typed

Question 4

JavaScript Datatypes

Answer 4

Number
String
Boolean
Object
Undefined

Question 5

== and ===

Answer 5

double compares value, triples compare datatype then value

Question 6

XML

Answer 6

extensible markup language, rule for encoding documents in format that is human- readable and machine readable

Question 7

Var

Answer 7

function scope

Question 8

Let

Answer 8

Block scope

Question 9

Const

Answer 9

block scope, only primitive is constant, can’t be changed once assigned. Need to be assigned when declared

Question 10

Falsy Value

Answer 10

undefined, null,0 , false , NaN, ‘ ‘

Question 11

Undefined

Answer 11

uninitialized variable

Question 12

Synchronous

Answer 12

same waiter serve multiple table but waits for chef to finish before server another

Question 13

Asynchronous Single threaded non-blocking

Answer 13

same waiter serving multiple table

Question 14

Closures

Answer 14

Closure is a locally declared variable related to a function which stays in memory when the function has returned. (e.g nested function)

Question 15

Promises

Answer 15

pending(initial state), fulfilled(operation success), rejected (operation failed), lets asynchronous method return values like synchronous method, instead of immediately returning the final value, it returns promise that it’ll supply the value in the future

Question 16

Hoisting

Answer 16

move var variables & function declarations to the top

Question 17

Type Coercion

Answer 17

convert value from one type to another (e.g. string to number)

Question 18

This keyword

Answer 18

In method: owner object
Alone: global object
In a function: global object
Event: element that received the event

Question 19

Limitation of JavaScript

Answer 19

Client-side

Can’t be used for networking applications

Question 20

Is Undefine and Null equal?

Answer 20

Undefine and Null are equal in value but different in type

Question 21

What does NULL mean in JavaScript?

Answer 21

The NULL value is used to represent no value or no object

Question 22

Callbacks

Answer 22

Function passed as an argument to another function. Used when a function have to wait for another function.
- Used for Asynchronous function (e.g. making HTTP request)

Question 23

What does the async function do?

Answer 23

Make a function return a promise.